When signals are transmitted among a plurality of semiconductor chips having different power supply voltages, direct transmission of signals through lines causes a difference in DC voltage, which may result in damage to the semiconductor chips and failure of signal transmission. Accordingly, when signals are transmitted among a plurality of semiconductor chips having different power supply voltages, the semiconductor chips are connected with an AC coupling element to transmit only AC signals. Examples of the AC coupling element include a capacitor and a transformer. Here, the transformer refers to an AC coupling element including a primary coil and a secondary coil which are magnetically coupled together. When the transformer is used as the AC coupling element, a turn ratio between the primary coil and the secondary coil of the transformer is adjusted. This allows transfer of signals with an appropriate voltage amplitude to the semiconductor chip on the reception side, regardless of the voltage amplitude of a transmission signal from the semiconductor chip on the transmission side. Thus, the use of the transformer in communication between the semiconductor chips, which operate at different power supply voltages, eliminates the need to adjust the voltage amplitude of the transmission signal or reception signal on the semiconductor chips. Hereinafter, the transformer formed on a semiconductor chip is referred to as an on-chip transformer, as needed.
Examples of a signal transmission technique using transformers are disclosed in Patent Literatures 1 to 8. In the signal transmission methods disclosed in Patent Literatures 1 to 5, two transformers are used for signal transmission. When a data value transits from a first value to a second value, a pulse signal is sent to a first transformer, and when the data value transits from the second value to the first value, a pulse signal is sent to a second transformer.
In the signal transmission methods disclosed in Patent Literatures 1, 2, and 4 to 6, consecutive pulse signals are sent to transformers during a period in which data has the first value, and the signal levels of the signals to be sent to the transformers are fixed during a period in which the data has the second value.
In the signal transmission methods disclosed in Patent Literatures 1, 2, 4, and 5, consecutive pulse signals each having a first frequency are sent to transformers during the period in which the data has the first value, and consecutive pulse signals each having a second frequency are continuously sent to the transformers during the period in which the data has the second value. Further, in the signal transmission methods disclosed in Patent Literatures 1, 2, 4, and 5, two transformers are used. During the period in which the data has the first value, the same signal is sent to the two transformers, and during the period in which the data has the second value, signals having inverted phases are sent to the respective transformers.
In the signal transmission method disclosed in Patent Literature 7, when the data value transits from the first value to the second value, a signal having one pulse is sent to each transformer, and when the data value transits from the second value to the first value, a signal having two consecutive pulses is sent to each transformer.
In the signal transmission method disclosed in Patent Literature 8, when the data value transits from the first value to the second value, a pulse signal having a first amplitude is sent to each transformer, and when the data value transits from the second value to the first value, a pulse signal having a second amplitude is sent to each transformer.